This invention relates to laser wire bonding and more specifically relates to an optical process monitor including a feedback control mechanism by which the reflectivity change of a wire being bonded is used for controlling the laser process parameters during laser wire bonding.
Laser wire bonding has been used in the past to join metal workpieces to each other and specifically in electronic circuit packaging for joining contact pads with fine wires or ribbons.
The bonding process involves providing laser energy through an optical fiber to a capillary bonding tip. The laser energy heats up the tip and under the combined influence of the localized heating and applied pressure urging the wire into intimate contact with the pad, the wire bonds to the pad. One such apparatus for use in practicing the laser bonding process is described in U.S. Pat. No. 4,534,811, issued to N. G. Ainslie et al, entitled "Apparatus for Thermo Bonding Surfaces" and assigned to the same assignee as the present application.
Since the bonding process is dependent upon the thermal effects caused by the laser energy, it is important to control certain variables or parameters of the laser energy, such as the power level and duration of the applied energy. Excessive heating can result in excessive melting of the wire or in cracks due to thermal stress in the ceramic substrate supporting the contact pad. Insufficient heating will result in poor bonding. Since the thermal environment of the pads may vary from one location on the substrate to another, it is not always possible to obtain adequate bonding of all the pads at different locations using a predetermined laser power level and laser pulse duration. For example, when bonding wires to pads disposed on a glass ceramic substrate, twice as much energy is required when bonding to a pad located directly above a via compared to bonding to a pad not over a via. The energy difference is a result of the heat sinking effect of the high thermal conductivity copper found in the via. Since the via location under a pad is not often well defined, it is important to have a monitor to control the laser energy for bonding at different pad locations.
Previous methods of bond monitoring include the use of a thermocouple sensor which measures the instantaneous temperature at the bond site. Such an arrangement is described in the article "Discrete Wire Bonding Using Laser Energy" by P. Chalco et al, Proceedings of the 1987 International Symposium on Microelectronics, Sept. 28-30, 1987, pages 435 et seq.